The invention relates to a centrifugal pump with an impeller which has a single blade of spiral shape.
Centrifugal pumps of this kind are disclosed by DE 26 42 231 A1 and DE 28 55 385 B1, among others. The housing and impeller of the previously known centrifugal pumps are matched to one another such that the blade edges form with the housing wall a narrow gap that remains uniform over the entire length of the blade and over the entire rotation of the impeller. Through this gap separating the discharge side from the suction side of the blade a leakage dependent upon the gap width necessarily flows while the pump is running. In a new pump before wear sets, in the loss produced at the edge of the blade and the housing wall is slight.
But since centrifugal pumps of the kind described herein are used chiefly for pumping media containing solids, severe wear at the long, revolving blade edge is unavoidable in many of the applications involved. If, for example, impurities that produce abrasion are contained in the fluid, then even after the centrifugal pump has been in operation but a short time wear is produced which appreciably lowers the original good efficiency of the pump. Such a danger exists in the pumping of communal sewerage, for example, in which the impurities are essentially uncontrollable. For example sand and similar frictionally acting components of the sewage cannot be excluded. The consequence is that centrifugal pumps used in this manner, whose pumping qualities are not tested or inadequately tested after installation, will operate over a long period of time with a steadily diminishing efficiency. This means, however, that such pumps will require unacceptably large power consumption to carry on the tasks assigned to them.
One danger of the previously known centrifugal pumps just described lies in the seizing of the impeller in the housing, which is caused by impurities which can get into the gap that has been enlarged by wear between the blade edge and the housing wall.
Now, impellers of the known kind just described are also provided with a front shielding disk and are thus rendered insensitive to sand abrasion. Certainly the cost, especially for the material, then becomes very high, since in the case of the conical vertical center sections here existing an annular gap must be formed between the shielding disk and the pump housing surrounding the latter. This annular gap, however, has to be kept relatively large, so as to avoid seizing at that location too.
It is the object of the invention to provide an improved centrifugal pump of the kind referred to above.
Another object of the invention is to provide a centrifugal pump in which the cost of the impeller and housing are low.
A further object of the invention is to provide a centrifugal pump which has a very low sensitivity to abrasive components in the fluid being pumped.
It is also an object of the invention to provide a centrifugal pump in which the danger of the seizing of the impeller in the pump housing will be avoided.
These and other objects have been achieved in accordance with the presently claimed invention by providing a centrifugal pump.
The problems of the prior art centrifugal pumps are overcome by the present invention due to the fact that the impeller has a shielding disk arranged on the suction side, which changes in a curve from an axial trend at the suction port of the impeller to a radial trend, that in the portion of the blade""s angle of entry in which there is a danger of cavitation in the event of an abrupt incident flow, the blade""s angle of entry is at least 5xc2x0 smaller than the angle of incident flow, that thereafter the blade angle in the axial portion of a radial section passing through the impeller increases at least to the magnitude of the blade""s exit angle, and finally a blade angle exceeding the blade""s exit angle in the radial portion of the vertical center section returns again to the magnitude of the blade""s exit angle.
The shielding disk used in the centrifugal pump according to the invention forms with the pump housing a gap through which a flow passes radially, and which can be made relatively short and narrow. Thus the problems involved in a conical annular gap are avoided.
The danger of seizing is also avoided, since the curved shielding disk made in accordance with the invention can be surrounded by an ordinarily configured wheel side chamber whose wall is at a relatively great distance from the shielding disk.
The cost of material and manufacture of the impeller of the invention, and of the pump housing, remains low.
The invention makes allowance for the circumstance that single impellers are basically very sensitive to cavitation, by providing for an especially shaped axial entry portion of the impeller. For if the blade""s angle of entry is made substantially smaller in the area in danger of cavitation by abrupt incident flow, then cavitation is avoided.
The blade angle in the area adjoining the impeller entry must necessarily increase considerably. In order to realize the transition from the axial shape of the impeller to the radial shape, the blade angle in the above-mentioned area in danger of cavitation merges in the radial part, after a steep rise in the axial part of the impeller, with the blade""s exit angle.
If the rise created in the axial part exceeds the magnitude of the blade""s exit angle, this means that in the radial part the blade angle has to return to the magnitude of the blade""s exit angle.